1. Field of the Invention
The present invention generally relates to a method and a device for image contrast enhancement and, more particularly, to a method and a device for image contrast enhancement by defining luminance signals as compensation parameters.
2. Description of the Prior Art
To date, there exist many conventional approaches to image color enhancement based on the information distribution of luminance signals in a frame histogram, which is a relationship diagram with the amount of corresponding luminance values in an image.
Please refer to FIG. 1, which is a circuit diagram showing a conventional method for extracting information in the histogram. In FIG. 1, the value of luminance signal (Y) can be figured out by the statistic histogram extracting device 110. Moreover, luminance distribution information 115 corresponding to the luminance value can be obtained in light of the statistic histogram. Due to the luminance distribution information 115, the luminance curve (the relationship curve between luminance and luminance value) device 120 outputs the luminance curve of a new luminance signal (Y′).
Accordingly, among the luminance signal (Y), the chroma signal (U or C1), and the chroma signal (V or C2) of every frame in an image, only the luminance signal (Y) is transformed into a new luminance signal (Y′) by means of the statistic histogram extracting device 110 and the luminance curve device 120.
In the prior art method, even though image contrast is improved, only the luminance is changed but the chroma is still the same, which leads to the loss of image saturation under some situations. Therefore, blurs occur in images near the interface boundaries between picture patterns, i.e., the side effect.
A conventional method for preventing the side effect is using a statistic histogram extracting device to obtain a new luminance signal and a compensating function to compensate chroma signals. Please refer to FIG. 2, which is another conventional circuit diagram for image contrast enhancement. In FIG. 2, not only the original luminance signal (Y) passes through a statistic histogram extracting device 112 and a luminance curve device 120 as to obtain a new luminance signal (Y′), but also the original chroma signals (U or C1) and (V or C2) pass through a color compensating device 210 as to obtain new chroma signals (U′ or C1′) and (V′ or C2′) according to the luminance signal (Y) and the new luminance signal (Y′). The original chroma signals (U or C1) and (V or C2) of every picture pattern in an image can be compensated as to overcome the side effect by using the compensating function of the color compensating device 210.
However, in the prior art method, the image luminance may not meet the viewer's requirements. Therefore, there is need in providing a method and a device for image contrast enhancement such that the image luminance can be adjusted simultaneously under the consideration of image chroma, so as to present better image luminance and image chroma to satisfy the viewer's requirements